The present invention pertains to a porosimeter for use in monitoring the porosity of a web of material, and pertains more particularly to such a porosimeter that is capable of cleaning itself periodically to prevent clogging by dust, etc.
It is frequently necessary to maintain the porosity of a web of material constant at a desired value. One application in which this is especally important is the manufacture of cigarette paper, and especially ciagarette tipping paper. In recent years it has become a common practice in the cigarette industry to provide Bands of fine perforations in the paper used to secure the filters to the cigarette rod, each band typically comprising one or more rows of perforation. If the porosity of the tipping paper is not correct within a very narrow tolerance, a cigarette made with the paper will not have the desired characteristics. One system for perforating tipping paper is disclosed in U.S. Pat. No. 4,247,754, issued Jan. 27, 1981, to Baier, for "Apparatus for Perforating Webs of Wrapping Material for Rod-Shaped Smokers Products", and a similar system, including a porosimeter and feedback loop to control the operation of the system, is disclosed in U.S. Pat. No. 4,025,752, issued May 24, 1977, to Whitman, III for "Apparatus for Electrically Perforating Dielectric Webs." Another example of a porosity measuring apparatus is that disclosed in U.S. Pat. No. 4,246,775, issued Jan. 27, 1981, to Stultz, for "Porosity Measuring Apparatus and Perforating System Using Same," assigned in common with the present application and the disclosure of which is incorporated herein by reference.
One type of porosimeter frequently used with tipping paper includes a chamber having a flat surface in which one or more apertures are provided, the web to be tested being pulled across the apertured surface while suction is applied to the apertures. By comparing the pressure in the chamber during this process with the chamber pressure in the absence of the web, it is possible to determine the porosity of the web. One porosimeter of this type is disclosed in U.S. Pat. No. 4,246,775, cited above.
One problem that has been encountered with porosimeters of this type is the accumulation of dust from the web in the porosimeter. The gradual accumulation of such dust can clog the suction lines of the porosimeter, detracting from the accuracy of the device. Debris on the porosimeter surface can also tend to make the location of the web transversely of the direction of motion unstable. It is desirable to provide a porosimeter which avoids this problem altogether. Some known porosimeters also tend to be inaccurate or imprecise when the speed of the web across the porosimeter surface is high or the web tension fluctuates excessively.